For many years, designers of pleasure craft have been researching water-borne craft designs which have aerodynamic elements generating lift force on the craft structure when at speed to assist raising the craft above its at rest buoyant draft. This is desired to reduce viscous drag on the craft hull providing increase speed and/or efficiency over a craft with a conventional design having the same weight and power. Early designs effectuating this objective are evidenced by the hydroplane style boat which comprises a pair of structurally supported sponsons separated by a raised hull floor, which form an underlying tunnel through which air is flowed. The underflowing airstream may also provide a lift force through the force of the airstream against the hull floor or through compression of the entering air mass between the water surface and the hull floor of the boat, or both.
The hull of a hydroplane is supported by buoyant force generally along its entire underside, including the hull floor and sponsons, when at rest. Under power the boat is substantially supported by hydrodynamic force acting against the outwardly and forwardly positioned sponsons, and by radial thrust from the prop extending from the rear of the null. This triangular footing provides good stability in smooth surfaces water, however, the wide beam of the sponsons and the airflow through the centrally defined tunnel can result in instability in rough surface conditions.
Generally, hydroplane style boats are designed with the cockpit in opposition to the powerplant, i.e. the cockpit is in the front if the engine is in the rear and vice versa, for heavier designs. For lighter designs, for instance boats under fifteen foot in length, the cockpit is generally positioned in the stern so that the pilot is within reach of an outboard type motor mounted to the transom of the boat.
Tri-hull boat designs exemplified for instance in U.S. Pat. No. 3,952,678, have been described as improvements over the basic hydroplane style hull. Such boat designs comprise a third water borne hull generally positioned centrally between the sponsons to assist in dynamic stability. The depths of the hull and sponsons are generally equal, as shown, to provide a uniform and broad base support for the hull in the water, with their underlying surfaces being flat and horizontally disposed.
The third central hull is thought to provide additional stability in rough surface conditions or high wind conditions which may create an unstable situation for a hydroplane style boat. Additionally, a forwardly rising support structure for the sponsons is described which provides an upwardly and forwardly angled undersurface which is bounded by the central hull and the respective sponsons. This undersurface is used to compress an airstream received when the boat is in motion to provide aerodynamic lift force on the underside of the structure in addition to the hydrodynamic lift force generated on the hull when the boat is under power. The boat described in the referenced patent appears to be larger than 20 feet in length and has the cockpit positioned rearwardly with the engine placed in an opposing forward position.
Another differing style craft which is substantially aerodynamic in design while utilizing both aerodynamic and hydrodynamic lift forces during the transitional period from standstill through surface departure, is described in U.S. Pat. No. 3,190,582 and related U.S. Pat. Nos. 3,627,235 and 3,030,448. The aircraft disclosed therein is, at rest, supported on forwardly and outwardly extended sponsons joined to a central craft fuselage. The sponsons are joined to the fuselage by airfoil shaped structures or wings extending outwardly and downwardly to the sponsons from the fuselage to provide a reverse dihedral wing configuration. This design positions forward portions of the fuselage above the sponsons such that the forward portion of the fuselage cannot contact the surface of the water on which the craft is supported. The wing structures extend rearwardly from the outwardly and downwardly directed leading edge which extends substantially perpendicular with the longitudinal axis of the fuselage, to an inwardly swept back rearward converging at the tail of the fuselage. The rearward edge of the wing sections extend in a generally horizontal plane from the sponsons to the rear of the fuselage. This wing configuration provides a triangular shaped frontal opening from the nose of the fuselage to the interior side of each respective sponson to define an underlying space below the fuselage and wing structures which is closed at the rearward edge of the wing against the surface of the water. The rearward edge of the wing is generally at the same vertical height as the sponsons and meets the water surface at rest from the sponson to the rear of the fuselage. Thus at rest the craft rests on the sponsons and the rearward edge of the wing and the rearward end of the fuselage, all of which are in contact with the water to support the aircraft.
When the aircraft begins operation and accelerates, the air flow into the triangular shaped frontal opening of the wing begins to build air pressure under the aircraft, between the undersurface of the wings and fuselage and the surface of the water. Maximum aerodynamic pressure builds at the rearward edge of the wings so that the rear of the aircraft lifts from the surface of the water first and the aircraft is supported by hydrodynamic pressure on the sponsons and aerodynamic pressure along the rearward edge of the wings.
Operation of the aircraft as velocity increases becomes increasingly unstable however due to loss of aerodyanmic lift as the rear edges of the wings rise and the ram air and ground effects lift dissipate. This causes great difficulty in pitch or attitude control of the aircraft. Due to the reverse dihedral configuration of the wings roll of the craft in one direction or the other tends to increase rotation of the same direction. This is caused by increases lift on the rising (more horizontal) wing as compared to the other, a phenomena which additionally causes attitude and roll instability.
If the aircraft is piloted through the transitional period, the aircraft attains a stable and substantially horizontal pitch attitude and airflow over the wings generates aerodynamic lift to raise the craft from the water surface into free flight.
The aircraft fuselage is configured in a common design having the cockpit positioned as far forward as is practical in view of other major components contained in the fuselage, such as engine, avionics, etc. which are positioned in the nose structure of the craft.
A watercraft comprising a singular water borne hull which additionally utilizes a wing(s) for stability and control in operation is known as a Ski Plane.RTM. which is manufactured by a concern known as Ski-Plane, Inc. of Newport Beach, Calif. The hull of the Ski Plane is a narrow cigar-shaped structure which has a primary substantially flat and narrow undersurface extending the length of the hull. A pair of secondary and adjacent horizontal undersurfaces are disposed on either side and are part of the hull, beginning with a raised surface portion approximately 1/3 along the length of the hull from the front and curved downwardly and rearwardly to a flat undersurface contiguous with the primary undersurface approximately midway along the length of the hull. The secondary undersurfaces are generally provided to aid high speed stability while decreasing the area of undersurface in contact with the water to reduce viscous drag.
A pair of wing structures extend laterally from the rear of the craft and exhibit a slight dihedral angle with the hull. Each wing structure ends with a downwardly curved portion or "drooping edge" which acts to restrict lateral flow of air from beneath the wing to improve stall characteristics, i.e. reduce the speed at which stall occurs. The wing structure ends do not, however, meet the water surface at level operation. Ailerons extend along the rearward edge of each wing structure to assist in rotational control of the craft when at speed. A fixed laterally extending winglet is also provided at the nose of the craft. The wing structures are thus characterized by a design common to an aircraft, rather than a waterborne vehicle.
The Ski Plane is powered by a typical outboard motor mounted to the transom of the hull to propel the craft and generate primary rotational control through a driving propeller disposed below the surface of the water. A pair of cockpits are provided in a generally forward position of the hull. Major control, fuel and drive components are mounted within the stern in the area where the wing structures are attached.